Several hundred varieties of cheese have been described. Each cheese variety is manufactured from milk using specific methods to give it unique flavor, texture, and functional properties. These methods include milk type, culture strain, cheese moisture content, salt content, etc. By adjusting these parameters, the cheese can be made with the desired properties. The manufacture of cottage cheese curd will be given special treatment here because it is particularly suited to the practice of this invention and will serve as a model system to demonstrate the invention's superiority over existing methods.
A brief description of the manufacture of cottage cheese will follow: Raw milk is delivered to the processing plant, where it is stored in a milk silo. When processing is to begin, the milk is moved through a transfer line to a balance tank. From the balance tank, the milk is sent to the regeneration side of a pasteurizer where it is exposed to one side of a two-sided heat exchanger and warmed to approximately 115.degree.-165.degree. F. The milk is then diverted to a cream separator where the butterfat is removed from the milk in the form of cream. The resultant skim milk is then passed through the heating side of the pasteurizer where it is heated to the desired temperature (about 162.degree. F.) and then into a holding tube where it is held at the temperature for the desired period of time (about 16 sec.). The holding time may be controlled by the flow rate through the pasteurizer. The skim milk passes from the holding tube into the cooling plates of the pasteurizer where the temperature is reduced to approximately 88.degree. F. The skim milk is then moved through a transfer line to the cheese vat. At the beginning, middle, or end, of fill, starter bacterial cultures (Streptococcus lactis or Streptococcus cremoris) are added to the milk at pH between 6.50 and 6.80. After inoculation, a coagulator preparation is added to the milk. This preparation is a very dilute proteolytic enzyme extracted from calf stomach or derived from microbial cultures. A small amount (4 to 12 mL/1000 lb. of milk) is all that is necessary because acid development and not enzyme activity is the driving force behind coagulation of the milk that occurs during the next 3 to 6 hours.
During this period the culture converts the mild's lactose into lactic acid via fermentation. This will eventually result in a drop in pH down to 4.70. During the drop in pH, the milk coagulates into a gel. At a pH of approximately 4.70, the coagulum is cut with harps into small cubes so that whey may be expelled. The curd is then allowed to settle t the bottom of the vat without stirring for 30 minutes. The heat is then turned on and the agitators started. The curds and whey are cooked to a temperature between 115.degree. C. and 145.degree. C. The final cook temperature is determined by the curd firmness. During the cook period it may be necessary to add cooking acid (usually phosphoric) to prevent matting of the curd. When the curd reaches the desired firmness, the whey is drawn off and 90.degree. F. water is introduced into the vat. The curd is rinsed for 10 to 15 minutes and the rinse water is drained off. A second rinse at 70.degree. F. follows. The second rinse is followed by a third and final rinse of 45.degree. F. The last rinse is drained and the curd is trenched. The trenched curd is allowed to drain for 30 minutes. Cream dressing is then introduced into the vat until the curd is sufficiently covered. The curd and cream dressing is agitated until adequate dispersion is achieved. The produce is then packaged.
Most cheese milk is heat treated (pasteurized). This is done so that pathogenic organisms which may be present in milk are eliminated. Another reason is to reduce the number of non-pathogenic organisms naturally present in milk so that they are not able to compete with the culture used to make the cheese. Cheese milk is usually pasteurized at the lowest temperature and held for the shortest time possible that will allow accomplishment of these goals (usually 162.degree. F. for 16 sec.). This policy is especially adhered to in the manufacture of cottage cheese curd. Higher temperatures and longer hold times often result in damage to the milk proteins and shifts in the milk's salt balance which can make cheesemaking difficult and unpredictable. For example, U.S. Pat. No. 3,316,098 (Noznick and Bundus, 1967) describes a system using extreme temperature and holding times to effect whey protein incorporation into the curd. Recommended temperatures and holding times were 185.degree. D. for at least 15 minutes; 255.degree. D. for 15 seconds; and 300.degree. F. for one second or less. The invention found little commercial acceptance because the cheese was of poor quality and the temperatures and holding times were too severe.
It has been found, however, that under the proper conditions and if carefully controlled, good quality cheese of high yield can be derived form milk which has undergone severe heat treatment and was coagulated with proteolytic enzyme. U.S. Pat. No. 4,959,229 (Reedy, et al., 1990) teaches that enhanced curd yields may be obtained through a combination of acid and heat. The process consists of a preacidification step followed by pasteurization of the milk at a temperature of 180.degree. F. to 190.degree. F. The heating of the preacidified milk results in complexing of the whey proteins with casein and increased curd yield. A subsequent postacidification is done to condition the proteins so that melt and stretch properties of the curd are improved. This process has been successfully applied to manufacture of mozzarella cheese.
Coagulation of milk for cheesemaking can be accomplished in two different ways, both of which are dependent upon a single chemical mechanism: the formulation of hydrophobic interactions between the among proteins. However, the route that rennet coagulation and acid coagulation take respectively to promote protein hydrophobic interaction are distinct and this distinction is important to the practice of this invention.
Rennet is a proteolytic enzyme preparation of animal or microbial origin whose substrate is casein, the primary protein in milk. In its native state, casein is comfortably suspended in milk and will not coagulate under normal conditions. It is relatively hydrophilic (water loving) in character. When rennet is added to milk the enzyme imposes structural changes on the casein which render it hydrophobic (water hating). This is a very rapid chemical conversion and for this reason rennet coagulation can be accomplished in a matter of minutes. The resultant coagulum is quite firm and resilient and will expel whey rapidly when cut. For this reason, rennet coagulation is sued in the production of low moisture hard cheese such as cheddar and mozzarella.
Although acid coagulation and rennet coagulation of milk are similar in their dependence on the formation of protein hydrophobic interactions, the chemical processes required to develop these bonds are quite different. Whereas rennet coagulation results from enzymatic modification of the structure and behavior of casein, acid coagulation is dependent upon casein reaching its isoelectric point. The process by which the casein reaches its isoelectric point is important to the teaching of this patent.
At the normal pH of milk (6.50-6.80) casein has a net negative electric charge. It is this negative charge that allows casein to be in a stable suspension in milk at milk's normal pH. When a lactic bacterial culture is introduced into the milk and allowed to ferment the lactose to lactic acid, the hydrogen ion concentration will increase and the pH will correspondingly drop eventually to the isoelectric point of casein. The heart of this chemical process is the gradual neutralization of the casein negative electrical charges to the point where negative charges are equalled in number by positive charges. This pH is the isoelectric point and is generally defined as the pH at which a protein has no net charge, has a strong repulsion to water, and an affinity for itself. The isoelectric pH of casein is approximately 4.70.
The process by which acid fermentation of milk causes coagulation is slower than the process of rennet coagulation and the resultant coagulum is more fragile due to less complete aggregation of casein resulting in stability of the curd particles against fusion and moisture loss. When milk undergoes a server heat treatment to increase yield by the complexing of hydrophilic whey proteins with casein incorporating more protein from the milk in the final cheese, the acid-induced coagulation process is hindered. This impediment may be overcome by allowing sufficient time during fermentation for the casein molecules to aggregate into a matrix which will result in a cuttable curd and subsequent yield increase. The reduction or elimination of a postacidification step provides for a greater pH drop to be accomplished by acid fermentation (culture) which extends the casein aggregation time. Therefore, a high heat treatment of preconditioned cheese milk combined with the development of a stable molecular configuration during acid coagulation will allow higher solids recovery, less curd shattering, and higher curd yield.